Treatment of the surfaces of aluminium or aluminium alloys



United States Patent TREATMENT OF THE SURFACES OF ALUMINIUM 0R ALUIVIINIUM ALLOYS Alfred Norman Douglas Pullen, Grappenhall, near Warrington, and Ernest Derek Swann, York, England, assignors to The British Aluminium Company Limited, London, England, a British company No Drawing. Application October 28, 1953, Serial No. 388,908

Claims priority, application Great Britain October 28, 1952 8 Claims. (Cl. 134-42) This invention relates to treatment of the surfaces of aluminium or aluminium alloys, referred to herein generally as aluminium, for the purpose of removing foreign matter, such as oil and grease, therefrom.

Such treatments are required, for example, in the case of sheets or strips where the surfaces are later to be subjected to a treatment, for example, anodising, which cannot be carried out satisfactorily unless the surfaces are free from oil and grease and are easily wettable. Again, aluminium wires or rods which are to be used for welding purposes, for example, as an electrode in an arc welding process in which the space around the welding location is filled with an inert gas, such as argon, must first be thoroughly cleaned, not only by removal of the film of oil or grease normally present as the result of the manufacturing processes, but also by the removal of that thin surface layer of the metal which is now considered to contain a high proportion of any gas which may be present in the metal, as well as extremely fine particles of foreign metals, dirt and grease unavoidably introduced during the drawing operations. Another case where careful cleaning is essential is that of aluminium wire, rod and sheet to be used for decorative purposes and the production of artificial jewellery, such products usually being anodised, often with electro-brightening, after the cleaning operation. that such preliminary or cleaning treatments shall be capable of being carried out relatively rapidly and that they shall cause only a minimum loss of metal and the least possible alteration in the appearance of the surface of the sheet, wire, rod or the like.

It is generally desirable in all these cases Various preparatory treatments for aluminium surfaces are already in use but these are not wholly satisfactory for one reason or another. For example, oil and grease may be removed, without loss of metal or alteration of appearance, by means of an organic solvent, such as trichlorethylene, but the treated surface may still have solid particles adhering thereto or embedded therein and is wettable only with diliiculty. Again, a treatment which consists in immersing the surface in a hot solution of a caustic alkali, rinsing, quickly dipping in a dilute solution of an acid, usually nitric acid, and finally washing and drying, will remove oil, grease and solid particles and leave the surfaces in an easily wettable condition. However, the activity of the caustic solution is not readily controllable and, due to excessive etching, loss of metal may be high and there may be a marked alteration in appearance. Frequently, also the time required for carrying out the treatment is too long. A surface treatment which has been suggested for the purpose of cleaning welding wire or rod, consists in immersing the wire or rod in a solution of an alkali metal zincate, where an appreciable amount of the aluminium is removed by solution and replaced by a thin deposit of zinc, and in then dipping the wire or rod in nitric acid to remove the deposit of zinc.

It is known that if fresh surfaces of aluminium are con tinually immersed in a hot solution of caustic soda, for example, aluminium in the form of successive sheets or a continuous moving strip, a point is reached at which the initial clarity of the solution disappears and a deposit starts to form. At this stage the activity of the solution has fallen appreciably from its initial value and it is the usual practice then to add more caustic soda.

It is also known that the addition of simple salts of certain metals, such as iron, cobalt and manganese, to solutions of sodium hydroxide or sodium carbonate will increase slightly the rate at which aluminium is dissolved by these solutions, the increase in rate being about 10% or even less.

Now we have found that it is possible in a preparatory treatment for the surface of aluminium to achieve the desired results in a relatively short period of time while making use of a treatment solution the activity of which may be controlled within fairly narrow limits. This. solution is one comprising a hydroxide and/ or a carbonate of an alkali metal and sufiicient dissolved aluminium to ensure that a permanent deposit will appear in the solution, to which has been added a small proportion of cobalt in the form of a cobaltammine which is stable, at least in such an alkaline solution, at the temperatures at which the solution is to be employed.

Such an addition of a suitable cobaltammine is found to impart to the solution an activity equal to, or even exceeding, that of a similar solution containing only the hydroxide and/ or the carbonate of the alkali metal in the same relative proportions, and this activity can be maintained for a considerable period of time without further additions of chemicals, except possibly those required to replace constituents of the solution which are lost by mechanical means, e. g. by dragout.

The rate at which aluminium is dissolved in a solution of sodium hydroxide and/ or sodium carbonate which is; initially saturated with dissolved aluminium is increased; as much as 6 or 7 times upon the addition of a suitable cobaltammine as compared with an increase of about 10% upon the addition of a simple cobalt salt.

When it is desirable, as it is in many cases, that the cleaning solution according to this invention, shall be used hot, e. g. at temperatures of 50 C. or more, the cobaltammine selected is one which will be stable in the alkaline solution at the temperatures concerned.

Various cobaltammines have proved of some value for the purposes of this invention but the preferred complex compound is the trans form of dinitro-tetrammine cobaltic chloride, of which the formula is H (CO Cl (NO2)2 Hexammine cobalt nitrate is stable at the higher temperatures but has poor cleaning properties; pentamminechloro cobaltic dichloride and tetrammine-carbonate cobaltic nitrate are both decomposed at temperatures of 35 C.; and formato-pentammine cobaltic dinitrate and formato-pentammine cobaltic di-iodide decomposed at 50, while also being poor activators of the cleaning solution. There are numerous other cobaltammine complexes some of which are automatically excluded because they either decompose spontaneously or are expolsive. Others exist only in solution but these can be used if they are stable in the alkaline solutions employed (at the temperatures of use) and have a useful activating action.

The preferred complex compound referred to above is readily prepared, completely stable at temperatures up to about 70 C. and is not decomposed by alkaline solutions of the concentrations likely to be used.

In the cleaning solutions of this invention the concentrations of the hydroxide and/or carbonate of the alkali metal are relatively low and are not critical. There is no advantage, however, in increasing the hydroxide content above about 3.5% or the carbonate content above about the lower limits being about 1.8% and about 1%, respectively. The amount of metallic aluminium dissolved in the alkaline solution is adjusted in each case to be about one half of the total weight of the hydroxide and carbonate present.

The complex cobaltammine employed may be added in proportions lying in the range from about 0.01% to about 0.03%.

t is preferred in all cases to include in tne treatment solution a small proportion of a non-ionic wetting agent, the amount added depending upon the ty"e of agent used and the nature of the aluminium product being treated. The amount of that wetting agent which may be a sulphonated glycol which is marketed under the Registered Trademark Penetrax which should be added when sheet or strip is to be treated is in the range of 0.1 to 0.6%, and when welding wire or rod is to be treated this range may be reduced to from about 0.05 to 0.3%.

The optimum treatment temperature is about 40 C. but the temperature may be varied from 30 to 50 C. if desired. The time of treatment may vary, for example, from 45 seconds to 2 minutes, depending on the initial state of the surface to be cleaned and the amount of metal to be removed.

As examples of the way in which the invention may be carried into effect, there may be given the following:

An aqueous solution to contain 2% by weight of sodium hydroxide and 1% by weight of sodium carbonate is initially made up with about 80% of the required amount of water, and sufficient aluminium metal, preferably in the form of fine wire, is dissolved therein to load the solution with aluminium to the deposition point. The amount required is approximately half the total weight of the sodium compounds. The remainder of the water is then added together with 0.01 by weight of dinitritotetrammine cobaltic chloride-trans. and about 0.25% by weight of the nor.ionic wetting agent known as Penetrax" (both calculated on the total quantity of the solution).

This solution is employed at a temperature of 40 C. and commercial aluminium sheets or strip are immersed therein for about 45 seconds. The thin grease film normally present on the sheets or strip is completely removed with the minimum loss of metal and alteration in appearance, the cleaned surfaces of the metal being readily wettable.

The cleaned surface may be rinsed in hot or cold water as desired, and if the material being treated is of the type on which a dark surface deposit forms when treated in alkaline solutions, this should be removed by a quick dip in cold, dilute nitric acid (about 5% strength). if, on the other hand, the sheets or strip are to be subsequently anodised in an acid electrolyte, they may be transferred directly from the treatment bath to the anodising vat without difficulty.

A similar solution may be employed for cleaning aluminium, welding wire or rod, the amount of wetting agent being reduced to about 0.125% by weight and the wire or rod being immersedtherein for about seconds. The thin grease film, normally present on the wire or rod is completely removed together with a very thin surface layer of metal.

The cleaned wire or rod is rinsed in cold water, dipped in a cold dilute solution of nitric acid (about 5% strength), Washed in cold or hot water and finally dried.

Although reference has been made in the examples above to aluminium sheet, Wire or rod only, it is to be understood that sheet, wire or red made from aluminium alloys may also be treated as above set forth.

What we claim is:

1. A method of cleaning aluminium and aluminium alloy surfaces which comprises immersing said surfaces for a short period of time in an aqueous alkaline solution which is held at a temperature in the range of 30 to 50 C. and contains an alkali-metal hydroxide in an amount of between 1.8% and 3.5% by Weight, dissolved aluminium in an amount approximately half the weight of the alkali-metal hydroxide to ensure that a permanent deposit will appear in the solution, and the trans form of dinitrotetrammine cobaltic chloride in an amount in the range from about 0.01% to about 0.03% by weight.

2. A method as claimed in claim 1, wherein the period of immersion is in the range from 45 seconds to 2 minutes.

3. A method as claimed in claim 1, wherein the solution also contains a non-ionic wetting agent in an amount of about 0.25 by weight.

4. A method as claimed in claim 1, wherein the solution also contains a sulphonated glycol non-ionic wetting agent in an amount of about 0.25% by weight.

5. A method of cleaning aluminium and aluminium alloy surfaces which comprises immersing said surfaces for a short period of time in an aqueous alkaline solution which is held at a temperature in the range of 30 to 50 C. and contains an alkali-metal carbonate in an amount of between about 1% and 5% by weight, dissolved aluminium in an amount approximately half the weight of the alkali-metal carbonate to ensure that a permanent deposit will appear in the solution, and the trans form of dinitro-tetrammine cobaltic chloride in an amount in the range from about 0.01% to about 0.03% by weight.

6. A method as claimed in claim 5, wherein the period of immersion is in the range from 45 seconds to 2 minutes.

7. A method as claimed in claim 5, wherein the solution also contains a non-ionic wetting agent in an amount of about 0.25% by weight.

8. A method as claimed in claim 5, wherein the solution also contains a sulphonated glycol non-ionic wetting agent in an amount of about 0.25 by weight.

References Cited in the file of this patent UNITED STATES PATENTS 2,050,122 Reuss et al. Aug. 4, 1936 2,241,984 Cooper May 13, 1941 2,541,901 Zademach Feb. 13, 1951 2,596,092 De Benneville May 13, 1952 

1. A METHOD FOR CLEANING ALUMINIUM AND ALUMINIUM ALLOY SURFACES WHICH COMPRISES IMMERSING SAID SURFACES FOR A SHORT PERIOD OF TIME IN AN AQUEOUS ALKALINE SOLUTION WHICH IS HELD AT A TEMPERATURE IN THE RANGE OF 30* TO 50* C. AND CONTAINS AN ALKALI-METAL BYDROXIDE IN AN AMOUNT OF BETWEEN 1.8% AND 3.5% BY WEIGHT, DISSOLVED ALUMINIUM IN AN AMOUNT APPROXIMATELY HALF THE WEIGHT OF THE ALKALI-METAL HYDROXIDE TO ENSURE THAT A PERMANENT DEPOSIT WILL APPEAR IN THE SOLUTION, AND THE TRANS FORM OF DINITROTETRAMMINE COBALTIC CHLORIDE IN AN AMOUNT IN THE RANGE FROM ABOUT 0.01% T ABOUT 0.03% BY WEIGHT. 